Expectation about a ubiquitous network society that it is possible to obtain all sorts of information about social life without being restricted by when and where, swells out. In order to realize the ubiquitous network society, it is necessary to construct both public wireless networks provided by communication common carriers and wireless networks (private wireless networks) that are set up in private space such as families and offices and operated by users, and complements each other. As one of schemes that realize networks in environments where various wireless nodes (hereinafter simply referred as to “nodes”) are ubiquitous, multi-hop wireless communication systems (multi-hop wireless networks) that perform not only direct communications between nodes but also information transmission (data transmission) via other nodes attract attention.
Multi-hop wireless networks are characterized in that it is possible to flexibly configure the network topology, and the transmission of broad-based information is allowed without depending on infrastructure such as base station and backbone line.
By the way, relay methods of relay nodes in multi-hop wireless network, can be classified broadly into two kinds, that is, the Amplify-and-forward scheme and the Decode-and-forward scheme. In addition, the Decode-and-forward scheme is a method that the relay node decodes the received signal, and then relays (retransmits) after performing coding again on the received signal that is already decoded.
In multi-hop wireless networks, the data transmitted from the source node is relayed by the adjacent node and transmitted to the destination node, wireless links for transmission and reception are constructed between nodes, broadcast traffic, multicast traffic and unicast traffic occur based on traffic and the network configuration (see Non-Patent Document 1). Therefore, by simultaneously using all the wireless links, the transmission characteristic (the throughput characteristic) is improved (see Non-Patent Document 2).
However, in multi-hop wireless networks, since it is different to construct wireless links between nodes that simultaneously uses the same channel for transmission, both one problem that the rate of wireless link building and the occupancy rate of wireless link are low and another problem that the interferences from adjacent nodes occur by simultaneously using the wireless link of the same channel, happen, it is impossible to realize a highly-efficient multi-hop wireless network.
In order to solve the above-described problems, the inventors of the present invention proposed a MIMO multi-hop/mesh network that performs a bi-directional transmission as a wireless network that not only improves the rate of wireless link building and the occupancy rate of wireless link but also realizes the transmitting-interference avoidance and the receiving-interference avoidance of adjacent nodes (see Non-Patent Document 6).
The MIMO multi-hop/mesh network disclosed in Non-Patent Document 6 is a network that the transmitting node and the receiving node exist alternately, each node has a relay function, and the transmission/reception of the forward link and the backward link are simultaneously performed. The MIMO multi-hop/mesh network disclosed in Non-Patent Document 6 realizes the interference avoidance of adjacent nodes by spatially multiplexing signals of the forward link and the backward link and the transmitting/receiving weights of antennas.
It is clear from the analysis based on the numerical simulation by computer that the MIMO multi-hop/mesh network disclosed in Non-Patent Document 6 can improve the transmission capacity than multi-hop wireless networks employing conventional MIMO algorithm.
Here, the MIMO multi-hop/mesh network disclosed in Non-Patent Document 6 will be described briefly.
FIG. 1 conceptually shows a model of the MIMO multi-hop/mesh network disclosed in Non-Patent Document 6. The lower part within FIG. 1, shows a signal model of the network that focuses attention on adjacent links.
In the model of the MIMO multi-hop/mesh network shown in FIG. 1, a 3-element MIMO antenna is used, the multiplexing of one stream for the forward link and one stream for the backward link is performed, and the number of the total streams (K) which a certain node transmits/receives, becomes two streams (K=2). Therefore, with respect to one desired signal, there are three interference signals.
Here, in the case of setting KF as the number of streams in the forward link and setting KB as the number of streams in the backward link, in the MIMO multi-hop/mesh network shown in FIG. 1, when a condition represented by the following Expression 1 is satisfied, it is possible to apply the linear algorithm and the nonlinear algorithm to a general topology.M≧K+max(KF,KB)  [Expression 1]
In other words, in the MIMO multi-hop/mesh network shown in FIG. 1, a 3-element MIMO antenna (M=3, i.e., at least three antennas) will be needed for a bi-directional transmission that perform the multiplexing of one stream for the forward link and one stream for the backward link.
By the way, in recent years, the network coding is studied. The network coding has an advantage that it becomes possible to save resources of the network by performing the coding of transmission data (see Non-Patent Document 3 and Non-Patent Document 4). For example, the transmitting node codes transmitting signals for the forward link and the backward link as one transmitting signal by using the network coding, and it becomes possible to simultaneously transmit as the same data (the same transmission data), thus it is possible to realize the transmitting-interference avoidance from the same transmitting node.
As described above, according to the MIMO multi-hop/mesh network disclosed in Non-Patent Document 6, it is possible to realize the link multiplexing and the interference avoidance of adjacent nodes.
However, the MIMO multi-hop/mesh network disclosed in Non-Patent Document 6 has the following problems.
Problem (A)
In order to realize the link multiplexing and the interference avoidance of adjacent nodes, the necessary number of antennas per node is at least three, thus there is a problem that the number of antennas is more than the number of the multiplexing links.
Problem (B)
In order to decide the transmitting/receiving weights of antennas of each node, both the transmitting node and the receiving node need channel information, thus there is a problem that the processing becomes complex.